[drzen]

I'm no expert but it looks to me like variance. Your SD is about the same. Someone who knows more maths than me will be along soon but it looks like you simply have results from different parts of the same distribution, and ran a bit better than your true winrate at 25NL and possibly a bit worse at 50NL.

Edit: before one of those maths guys turns up and goes blah blah, variance is sd squared or whatever, blah blah, I'm using the common parlance here not a mathematical definition